The present invention relates to thermosettable vinyl ester/polyester resinous compositions (e.g. sheet molding compound or SMC) and more particularly to new low profile additives (LPAs) therefor.
Vinyl ester resinous compositions are liquid thermosetting resins which are the reaction product of about equal amounts of a polyepoxide and an unsaturated monocarboxylic acid. These resins often are used in combination with fibrous reinforcement and inert fillers to manufacture composite structures often called sheet molding compound or SMC. One way to make such composite structures is to pre-mix the resin, filler, fibrous reinforcement, and other additives to form the molding compound. The molding compound then can be formed into the desired shape and cured in a heated, matched metal dye. An improvement in the process is chemical thickening of the relatively low viscosity liquid resin, e.g. with a Group II metal oxide or hydroxide and water, to form a high viscosity gel after the resin has been mixed with all other ingredients in the molding compound. This thickening or B-staging has several advantages. Unthickened molding compounds are sticky masses which are difficult to handle. After B-staging, they are firm solids whose surfaces are dry. In this form, they can be handled easily. During the molding operation, the molding compound flows within the die set to fill the die cavity. The increased viscosity of B-staged molding compounds inhibits segregation of the various components of the molding compound during flow and promotes compositional uniformity of the composite over the entire volume of the structure.
The advent of low shrink-low profile additives has led to a considerable growth in sheet molding compounds and bulk molding compounds, such as described above. Previous to the development of these additives, reinforced molded parts had rippled or undulating surfaces, which required laborious sanding operations or other corrective measures to obtain painted parts with a metal-like appearance. The low shrink-low profile additives exhibited great benefits to providing exceptionally smooth surfaces.
Polyether polyols have been used as anti-shrink or low profile control additives in unsaturated polyester systems (U.S. Pat. Nos. 4,472,544 and 4,483,963). U.S. Pat. No. 4,472,554 describes the usage of a very high level of acidified polyethertriol as the shrinkage control additive. U.S. Pat. No. 4,483,963 describes the reaction product of oligoester with unsaturated polyester as the low profile additive (LPA) for unsaturated polyester systems. In vinyl ester systems, polyether polyols also have been reported as being useful as low profile additives (U.S. Pat. Nos. 4,151,219, 4,347,343, and 4,824,919). Most of the molecular weights in compositions of prior art polyethers have disadvantages. For example, their molecular weights tend to be too low for good shrink control and most are not compatible with vinyl ester resins.
Moreover, the low profile efficiency of most low profile additives is sensitive to the type of resin used. For example, Arakawa et al., "Factors Affecting Surface Smoothness and the Role of Unsaturated Polyester Resin and Glass Fiber", Section 1A, 47th Annual SPI Conference, February, 1992, report that the linear shrinkage of SMC is related to the reactivity of unsaturated polyester resin. The more reactive the resin is that is used in SMC, the less the shrinkage of the SMC. This result indicates that the molecular structure of the unsaturated polyester affects the low profile efficiency (or shrink control) of the LPA used in SMC. Lin et al., "A Study of Low Profile Additive on the Surface Morphology of Vinyl Ester SMC", Section 6E, 45th Annual SPI Conference, February, 1990, also report that one LPA used in the unsaturated polyester had very good shrinkage control while the same LPA used in vinyl ester resin did not provide good shrinkage control.